WASHINGTON  Global warming puts 12 of the most famous U.S. national parks at risk, environmentalists said Tuesday, conjuring up visions of Glacier National Park without glaciers and Yellowstone Park without grizzly bears.

All 12 parks are located in the American West, where temperatures have risen twice as fast as in the rest of the United States over the last 50 years, said Theo Spencer of the Natural Resources Defense Council.

"Rising temperatures, drought, wildfires and diminished snowfalls endanger wildlife and threaten hiking, fishing and other recreational activities" in the parks, Spencer said in a telephone news conference. "Imagine Glacier Park without glaciers or Yellowstone without any grizzly bears."

Most climate scientists believe Earth's surface temperature has risen over the last century or more, spurred by human activities that produce greenhouse gases, which trap heat like the glass walls of a greenhouse. Some skeptics doubt that people affect global climate change and say temperature fluctuations have occurred throughout history.

The report released by the council and the Rocky Mountain Climate Organization stressed the connection between global warming and environmental damage at the parks, including the loss of specific wildlife, and called on the U.S. government to cut greenhouse gas emissions significantly in 10 years.

The report blamed global warming for threatening grizzly bears, an iconic species in Yellowstone Park.

BEETLES AND BEARS

The bears feed on whitebark pine seeds, but global warming has encouraged beetles to infest whitebark trees that grow at high altitudes where grizzlies feed; cold weather would normally kill the beetles but this has not occurred in recent years, said Janet Barwick of the council's Wild Bears Project.

This in turn forces the bears to move to lower altitudes to look for food to fatten up for the winter, making them more likely to move into areas where there are people and that leads to an increase in grizzly mortality, Barwick said.

Glaciers and ice caves have melted in North Cascades and Mt. Rainier parks, and mountaintops in Western parks could be snow-free in summer within decades, said Stephen Saunders of the Rocky Mountain Climate Organization. He said all glaciers in Glacier National Park could be gone within 25 years.